Vessel for a metal gas cell

ABSTRACT

A support assembly, including an end plate, for use in metal gas cells. The support assembly inhibits a plate stack which is positioned within a pressure vessel from impinging the walls of the pressure vessel. The support assembly comprises a first and a second weld ring and a first and a second end plate, each of the end plates being attached to a different end of the plate stack. The first and second weld rings mate with the first and second end plates, respectively, and cooperate to support the plate stack. The end plate comprises two plates secured together. A raised portion on one of the plates forming the end plate has a radially extending embossment formed therein to increase the rigidity thereof. The pressure vessel is constructed of generally cylindrical container defining two circumferentially extending lips, each of which is secured to an end portion.

This is a divisional of application Ser. No. 07/465,694, filed Jan. 16,1990, now U.S. Pat. No. 5,002,842, which in turn is a divisional of Ser.No. 07/189,264, filed May 2, 1988, now U.S. Pat. No. 4,950,564.

BACKGROUND OF INVENTION

The present invention relates to a metal gas cell, such as a nickelhydrogen battery, and more particularly, to a support assembly for aplate stack which is utilized in a metal gas cell.

Metal gas cells, in particular nickel hydrogen batteries, have evolvedto include a plate stack encased within a sealed metal vessel. Thevessel has a generally cylindrical configuration and is charged with agas under pressure, such as hydrogen. Conventionally, a sheet of metal,such as a nickel alloy, is hydroformed into a hollow cylindricallyshaped member having one hemispherically configured end portion bystretching the sheet around an appropriately shaped mandrel. Two suchmembers are welded together to form a cylindrically shaped vessel orcasing having two hemispherically configured end portions. Theconstraints of the hydroforming process limit the overall length of thevessel, for example a maximum length of approximately 14 inches can beobtained for a 3.5 inch diameter vessel.

As conventionally manufactured, the length of the pressure vessel, andthus the capacity of an individual cell, has been limited by constraintsimposed by the process of hydroforming metal. Typical nickel hydrogenbatteries which have a cell stack cantilevered from one end thereof aretypically 8-12 inches in length. In addition, the high tooling costsassociated with hydroforming further constrains the vessel diameterwhich must be prescribed at the outset of the hydroforming process.Further, since the thickness of a hollow cylindrical shaped member whichis hydroformed for construction of the pressure vessel varies and issmallest at the segment of the member intermediate the hemisphericallyconfigured end portion and the lip or edge of the member, the vesselmust be designed to withstand a given burst of pressure using theminimum thickness thereby resulting in increased expense and vesselweight.

Components of a simple plate stack include a negative electrode plate, agas diffusion separator, and a positive electrode plate. Each of thesecomponents is a relatively thin, e.g., 0.01-0.05 inch, disc or annularwafer having an opening or aperture through the center thereof. Asassembled on an elongated core, the components are juxtaposed andaligned and are repeated in series to form the conventional plate stackof an individual metal gas cell.

Supporting a conventional plate stack within a sealed pressure vesselhas previously been accomplished by inserting an elongated, generallycylindrical core having an end plate attached near one end thereofthrough the aligned apertures of the components of an assembled platestack. A second end plate is releasably secured on the core in acontiguous relationship with the other end of the assembled plate stack.One end of the core is fixedly secured to the pressure vessel near oneend thereof, usually by a weld ring which is utilized to weld the twomembers together which form the pressure vessel. Thus, the plate stackis cantilevered from one end of the core and the pressure vessel. Sincea substantial portion of the pressure exerted by and weight of the platestack is sustained by the elongated core, the core must be redesignedfor each different size of cell in which it is employed so as towithstand the specific pressure and weight imparted to it. Further,since a nickel hydrogen battery functions as an energy source forsatellites, the forces transmitted to the cell or battery duringprelaunch testing, during launch and flight of the satellite dictatethat the plate stack of an individual battery be supported so as toprevent damage from such forces. During satellite testing, launch, andmovement in orbit, forces acting on a conventional battery can causeunsupported components of the plate stack, especially those componentsnear the unsupported end of the core, to impinge upon the walls of thepressure vessel thereby damaging and/or shorting the electrodes. Suchimpingement can result in premature cell or battery failure.

It has been suggested to cantilever mount two separate plate stacks froma weld ring which is positioned approximately in the center of apressure vessel to increase the energy capacity of a nickel hydrogenbattery, as well as the integrity and durability thereof. However, aninternal impedance to electrolyte communication between the two separateplate stacks is created by the centrally located weld ring.Additionally, each separate plate stack is cantilevered from a centralsupport and thus may be subject to premature cell failure due to theforces encountered during launch and orbit of a satellite.

End plates used in metal gas cells have been constructed of two annular,correspondingly sized plates having an opening or aperture through thecenter thereof which are spaced apart and fixedly secured together bymeans of a plurality of ribbons. The plurality of ribbons are secured toeach plate by any suitable means, such as by welding. The plates andribbons are constructed of a nickel based alloy. However, conventionalmetal end plates are expensive and require a relatively longmanufacturing lead time. And the relatively small rates necessary toproduce end plates for the metal gas cell market do not justify the highinjection mold costs associated with manufacturing plastic end plates.Thus, a need exists for an inexpensive, thin, e.g. 0.10 in., low volume,and light weight end plate for use in metal gas cells.

Accordingly, it is an object of the present invention to provide a metalgas cell wherein the plate stack which is enclosed within a pressurevessel is effectively supported along substantially the entire lengththereof.

Another object of the present invention is to provide a support assemblyfor a plate stack utilized in a metal gas cell in which an elongatedcore on which the plate stack is positioned is not a substantialstructural component for supporting the weight of pressure imparted bythe plate stack.

It is also an object of the present invention is to construct a metalgas cell having a length which is not constrained by the process ofmanufacturing the pressure vessel component thereof.

It is a further object of the present invention to provide a pressurevessel for use in a metal gas cell which has a substantially uniformthickness, is lightweight, and can be manufactured to any given length.

It is still a further object of the present invention to provide an endplate for use in metal gas cells which is relatively thin, lightweight,and inexpensive and has a relatively low volume and high strength.

SUMMARY OF THE INVENTION

To achieve the foregoing and other objects, and in accordance with thepurposes of the present invention, as embodied and broadly describedherein, one characterization of the present invention comprises asupport assembly used in a metal gas cell for inhibiting a plate stackwhich is positioned within a vessel containing a fluid at an elevatedpressure from impinging the vessel as a result of external forces actingon the cell. The support assembly is fixedly secured to the vessel atdistant locations and is connected to both ends of the plate stack. Theplate stack defines two ends and comprises a negative electrode plate, apositive electrode plate, and an electrically insulative, porousseparator plate positioned between the negative electrode plate and thepositive electrode plate.

In another characterization of the present invention, a first and asecond support assembly are provided for use in a metal gas cell. Thefirst and second support assembly cooperate to inhibit a plate stackwhich is positioned within a vessel containing a fluid at an elevatedpressure from impinging the vessel. The plate stack has a generallycylindrical configuration defining two ends and comprises a plurality ofcomponent plates. The first support assembly is connected to one end ofthe plate stack and is fixedly secured to the vessel, while the secondsupport assembly is connected to the other end of the plate stack and isfixedly secured to the vessel.

In yet another characterization of the present invention, a supportassembly is provided for use in a metal gas cell and comprises a firstand a second ring and a first and a second end plate, each of which havean aperture therethrough. A generally cylindrical configured plate stackhaving an aperture therethrough is positioned within a vessel. The platestack comprises a plurality of component plates including a negativeelectrode plate, a positive electrode plate, and an electricallyinsulative, porous separator plate positioned between the negativeelectrode plate and the positive electrode plate. An elongated core ispositioned within the apertures. The plate stack, the first end plate,and the second end plate are releasably secured to the elongated core.The first end plate mates with the first ring and the second end platemates with the second ring to inhibit the plate stack from impinging thevessel.

In still another characterization of the present invention, an end platefor use in a metal gas cell is provided which is comprised of a firstplate and a second plate. The first and second plates are securedtogether. The second plate has an inner and an outer circumference andan interposed, circumferentially extending raised portion. The raisedportion has a radially extending embossment formed therein.

In yet a further characterization of the present invention a vessel isprovided for housing components of a metal gas cell. The vesselcomprises a generally cylindrical container defining twocircumferentially extending lips, a first end portion secured to one ofthe lips, and a second end portion secured to the other lip.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate the embodiments of the present inventionand, together with the description, serve to explain the principles ofthe invention. In the drawings:

FIG. 1 is a partially cutaway, partially cross-sectional pictorial viewdepicting the support assembly of the present invention as assembledwithin a metal gas cell;

FIG. 2 is an exploded pictorial view of internal components of a metalgas cell including end plates and plate stack components as arranged forassembly onto an elongated core;

FIG. 3 is a partially cutaway side view of a prior art end plate for usein a metal gas cell;

FIG. 4 is an end view of a prior art end plate for use in a metal gascell;

FIG. 5 is a side view of the end plate of the support assembly of thepresent invention; and

FIG. 6 is a quarter sectional end view of the end plate of the supportassembly of the present invention taken along line 6--6 of FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, the metal gas cell of the present invention isillustrated generally as 10 and is comprised of a thin walled pressurevessel 12 which is capable of containing a fluid under a pressuresuitable for operation of the cell for example, gaseous hydrogen at600-1200 psi. Pressure vessel 12 is comprised of an intermediatecontainer portion 14 which has a generally cylindrical configuration andend portions 16 and 18 secured to each end of container portion 14 ashereinafter described. Container 14 is formed by rolling a sheet ofsuitable material into a cylindrical configuration and by fixedlysecuring abutting edges of the material together by any suitable means,such as by welding the edges together along the entire length ofcontainer 14. End portions 16 and 18 are hydroformed into a hollow,generally hemispherical configuration and are secured to separate endsof container 14 by any suitable means, for example welding, in a mannerhereinafter described. As thus constructed and assembled, vessel 12 canbe manufactured to any desired length for a given diameter. Vessel 12may be constructed of any suitable light weight material, such as,Inconel 718 which is a nickel alloy manufactured by the InternationalNickel Company.

A core member illustrated generally as 30 in FIGS. 1 and 2 onto whichplate stack 20 is positioned between end plates 50, 51, and 52 and weldrings 60, 62 comprises an elongated core 34. Plate stack 20 has agenerally cylindrical configuration defining two ends, has an aperturetherethrough, and is comprised of a plurality of component platesincluding a negative electrode plate, a positive electrode plate, and anelectrically insulative, porous separator plate interposed between thepositive and negative electrode plates. Core 34 has a generally annularperipheral cross sectional configuration, has a flange 32 integrallyformed at one end thereof defining an annular shoulder 33 therebetween,and a male threaded portion 36 formed at the other end of core 34. Coremember 30 may also have one or more troughs 38 formed in an outersurface and extending substantially the entire longitudinal lengththereof for conducting negative and positive tabs to form leads 70, 71as illustrated in FIGS. 1 and 2. Leads 70, 71 are connected to terminals80, 81 respectively as will be evident to the skilled artisan. Eachterminal 80, 81 is provided with a teflon compression feed through seal82 and a sealing collar 83 to seal vessel 12 against fluid leakage.

FIG. 2 illustrates the arrangement for assembling end plates 50, 51 and52, belleville washers 40, 43 and 48, and the component parts of theplate stack 20 on core member 30. As illustrated, belleville washer 48is positioned on elongated core 34 such that the inner diameter thereofabuts against shoulder 33. Thereafter end plate 52 is positioned onelongated core 34 such that the substantially flat surface thereof willabut one end of plate stack 20 while the embossed surface thereof abutsthe outer diameter of washer 48. Plate stack 20 is then positioned onelongated core 34. As illustrated in FIG. 2, plate stack 20 consists ofa gas screen 21, a negative electrode plate 22, two separators 23, aninsulating ring 24, positive electrode plates 25 and 26, an insulatingring 27, two separators 28, and negative electrode plate 29. Each ofthese components of the stack 20 is in the form of a annular wafer orplate having an opening or aperture through the center thereof forplacement of each component onto elongated core 34. Separators 23, 28are formed of any suitable electrically insulative material which hassufficient porosity to permit passage of a liquid electrolytetherethrough, for example a plastic such as polysulfone, polyamides,inorganic oxides, or asbestos. Components 21-29 can be repeated inseries a plurality of times depending upon the capacity of the cellbeing constructed as evident to the skilled artisan. Each of thenegative and positive plates are provided with a tab constructed of acurrent conducting material, for example nickel. Tabs transporting alike charge are stacked upon one another in a contiguous relationshipand are positioned within a trough 38 for connection to the positive ornegative terminal of the cell as appropriate and as hereinafterdescribed. A gas screen 47 which has a similar configuration to gasscreen 21 is positioned on elongated core 34 so as to be continuous withnegative plate 29. End plate 51 is then placed onto elongated core 34such that the substantially flat surface of end plate 51 abuts gasscreen 47. Belleville washers 40 and 43 are placed onto elongated core34 and separated by spacer elements 44, 45 and 46 which compensate formanufacturing tolerances of the individual components of the platestack. End plate 50 is then positioned on elongated core 34 such thatthe flat surface of end plate 50 is juxtaposed to belleville washer 43.Finally, the components which have been inserted onto core 34 arepressed together by application of suitable force on end plate 50 andnut 49 which has a female threaded bore therethrough is fully mated withmale threaded portion 36 of core member 30. A common failure mechanismof metal gas cells, such as a nickel hydrogen cell, is represented byrepeated charge and discharge cycles of the cell which cause thepositive electrodes within the plate stack to axially swell. In thesupport assembly of the present invention, such swelling compressesbelleville washers 40, 43, and to a lesser extent separators 23 and 28.

End plates 50, 51, and 52, weld rings 60, 62, and belleville washers 40,43, and 48 may be constructed of any suitable light weight material,such as, Inconel 718. Core member 30 and spacer elements 44, 45, and 46may be constructed of any suitable electrically insulative materialwhich has a relatively high strength and utility at relatively hightemperatures, for example a plastic such as polysulfone.

Referring now to FIGS. 1 and 2, the metal gas cell 10, particularly anickel-hydrogen battery, is assembled by initially positioning a weldring 62 between end portion 18 and container 14 and axially aligning thesame prior to welding weld ring 62 to end portion 18 and container 14from the exterior of vessel 12. A terminal 81 is fixedly secured, suchas by welding to lead 71 formed of the plate tabs emanating via a trough38 from the flange 32 at one end of elongated core 34, while a separateterminal 81 is fixedly secured to lead 70 formed of the tabs emanatingvia a trough 38 from threaded portion 36 at the other end of elongatedcore 34. The terminal 81 attached to lead 71 is inserted through theaperture formed at the end of end portion 18 and core 34 is insertedthrough the aperture in weld ring 62 until end plate 52 abuts and ismated with weld ring 62. Intermediate container portion 14 is sized suchthat when the metal gas cell of the present invention is partiallyassembled in this manner, weld ring 60 abuts the edge of container 14 oris spaced therefrom only by a distance through which weld ring 60 can beforced into contact with the edge of container 14. The unsecuredterminal 80 is positioned within the opening in the end of end portion16 and end portion 16 is positioned to abut weld ring 60. Thereafter,end portion 16 and weld ring 60 are axially aligned and are forced intocontact, if necessary, with container 14. Upon contact of end portions16 and weld ring 60 with vessel 14, weld ring 60 is welded from theexterior of vessel 12 to both vessel 14 and end portion 16. In thisassembled position, weld ring 60 abuts end plate 50 resulting in nut 49being spaced apart from weld ring 60 by an extremely small distance, forexample 0.001 inch. Belleville washer 40 partly serves to maintain endplate 50 in contact with weld ring 60. Terminals 80 and 81 are securedto end portions 16 and 18, respectively, by any suitable means, such asby compression. As thus fully assembled, weld rings 60 and 62 incombination with belleville washers 40 and 43 are subjected tosubstantially all of the pressure and weight imparted by the compressedplate stack. In contrast, elongated core 34 is substantially free fromany of the load imparted by the compressed plate stack and is restrainedfrom axial movement within the vessel by the relatively small pressure,e.g. 10-30 lbs., exerted on it by belleville washer 48. Depending oncell size, the drag of the plate stack components may be sufficient toprevent axial movement of core 34 thus eliminating the need forbelleville washer 48. This absence of load permits a single structuraldesign of elongated core 34 to be utilized within any metal gas cellincorporating the support assembly of the present invention subject onlyto varying the length as dictated by cell capacity.

As illustrated in FIGS. 3 and 4, prior art end plates for use insupporting the plate stack of conventional metal gas cells are comprisedof two axially aligned annular plates having a central aperture 92therethrough and fixedly secured together by means of a plurality ofstrips or ribbons 94 which are aligned in a parallel relationshipbetween the plates 90, 91 so as not to protrude beyond the periphery ofeither plate and are secured to each plate by means of welds aspreviously mentioned. As previously mentioned, prior art metal endplates are expensive to manufacture and have exceptionally longprocurement lead times.

The end plate utilized in the metal gas cell of the present invention isillustrated in FIGS. 5 and 6 and comprises a first substantially flatplate 53 having an axially aligned substantially annular aperture 55therethrough. A second plate 54 has a outer annular rim portion 56 andinner annular rim portion 57. Plate 54 is also provided with an axiallyaligned, substantially annular aperture 55 which is correspondinglysized and aligned with aperture 55 through plate 53. Plate 54 is alsosized to have an outer peripheral configuration which substantiallycorresponds with the outer peripheral configuration of plate 53 whenapertures 55 are aligned as illustrated in FIG. 5. Plate 54 is providedwith a raised intermediate portion between outer rim 56 and inner rim 57which has a series of radially extending grooves 58 embossed therein byany suitable means, for example, by hydroforming. Plates 53 and 54 aresecured together by any suitable means, for example, by a plurality ofspot welds uniformly spaced along the rim 56, rim 57 and grooves 58.Spot welds may be accomplished by any suitable means of welding, forexample, by laser welding or electron beam welding. A series of dimples59 may be provided between each radially extending finger 58 to increasethe structural integrity and rigidity of the end plate of the presentinvention as assembled. The exact number of radially extending grooves58 which are formed in plate 54 is preferably from 12 to 16. Whendimples 59 are formed between radially extending grooves 58 spot weldsmay be made at the approximate center of the dimple to further secureplate 53 to plate 54. A plurality of holes may be provided in the raisedportion of plate 54 to allow for drainage of any electrolyte which maybecome trapped between plates 53 and 54. While grooves 58 and dimples 59have been illustrated in FIGS. 5 and 6, any design can be embossed inthe raised portion of plate 54 which provides a uniformed stiffnessacross plate 54 due to the radial nature of the design embossed. Theembossed design inherently reduces the volume of the end plate of thepresent invention.

Weld rings 60 and 62 have a circumferentially extending outer rim 64 anda circumferentially extending inner rim 66 which are axially offset andconnected by means of rib 65. End plates 50, 52 and weld rings 60, 62are sized so that when assembled on core 34 and secured by nut 49, outerrim portion 64 abuts outer rim 56 and the circumferential outer edge ofthe raised immediate portion of each weld ring is contiguous with rib65. As fully assembled with a metal gas cell, end plates 50, 52 aremated with and nestled within weld rings 60, 62 respectively.

Thus, the present invention provides a support assembly including endplates 50, 51 and 52 and weld rings 60 and 62 which, as assembled,function in cooperation with elongated core 34 and vessel 12 to supporta plate stack of a metal gas cell while substantially reducing the loadimparted to the elongated core around which the plate stack iscontiguously positioned. The support assembly of the present inventionis secured to each end of the plate stack and cooperates with vessel 12so as to effectively support the plate stack along substantially theentire length thereof. Accordingly, the premature cell failure due toshorting of the electrodes is reduced since the plate stack is inhibitedby the support assembly of the present invention from impinging vessel12 upon the cell encountering external forces, such as the forcesencountered during launch and orbit of a satellite. Although the supportassembly of the present invention has been described as havingparticular utility in a nickel-hydrogen battery, the support assemblycan be utilized to support a plate stack within any metal gas cell,including silver-hydrogen and lead-hydrogen batteries.

The following example describes the manner and process of making andusing the present invention and sets forth the best mode contemplated bythe inventors of carrying out the invention but is not to be construedas limiting the scope thereof.

EXAMPLE

A nickel hydrogen battery was constructed to have an overall length of9.763 inches and a capacity of 88 ampere hours. A 4.307 inch long and0.024 inch thick cylindrical container was formed by rolling a sheet ofInconel 718, a nickel alloy manufactured by the International NickelCompany, into a cylindrical configuration and welding the abutting edgesof the sheet together by tungsten inert gas (TIG) welding. A firsthollow hemispherically configured end portion hydroformed from a sheetof Inconel 718 and having a length of 2.709 inches and a nominalthickness of 0.027 inches was assembled to one edge of the container byinterposing, axially aligning, and externally and circumferentiallywelding a weld ring by means of a TIG welder. The weld ring wasconstructed of Inconel 718 and had an inner diameter of 2.868 inches, anouter diameter of 3.550 inches, and a thickness of 0.019 inches. Anelongated core constructed of polysulfone and having a length of 4.737inches had a plate stack, belleville washers, end plates, and spacerelements positioned thereon in the manner described herein andreleasably secured thereto by means of a nut constructed of polysulfoneand mated with the male threaded portion of one end of the core. Eachend plate was constructed of Inconel 718 and had an inner diameter of0.789 inches and a thickness of 0.101 inches. The end platescorresponding to 51 and 52 in FIG. 1 had an outer diameter of 3.375inches while the end plate corresponding to 50 in FIG. 1 had an outerdiameter of 3.295 inches. Each end plate had the radial designillustrated in FIG. 5 embossed in one face thereof by hydroforming. Eachbelleville washer was constructed of Inconel 718 and had a thickness of0.040 inches, an inner diameter of 1.25 inches, an outer diameter of2.375 inches, and a full compression force of 150 lbs. Three polysulfonespacer elements were employed between the belleville washerscorresponding to 40 and 43 in FIG. 1. Each spacer element had an innerdiameter of 0.793 inches, an outer diameter of 1.375 inches, and athickness of 0.030 inches. Upon assembly, the elongated core, platestack, and support assembly were positioned within the welded cylinderand end portion. A second hollow hemispherically configured end portionwhich was identical to the first end portion in construction anddimension was assembled to the other edge of the container byinterposing, axially aligning, and circumferentially welding a secondweld ring by means of an TIG welder. The second weld ring wasconstructed of Inconel 718 and had a inner diameter of 2.982 inches, anouter diameter of 3.550 inches, and a thickness of 0.019 inches. As thusconstructed, the nickel hydrogen battery was designed to withstand thehigh levels of vibration and shock which are believed to be encounteredin satellite launch and flight, for example, a random vibration profileof 12.1 G rms and a pyrotechnic shock response spectrum.

As illustrated in FIG. 1, plate stack 20 as fully assembled andsupported within metal gas cell 10 in accordance with the presentinvention is not axially centered within the cell. In order to maintainthermal symmetry between a plurality of cells so that any heat transferdevice is independent of the relative orientation of the cells, acentering ring (not illustrated) can be interposed between weld ring 62and end plate 52. The centering ring possesses a thickness which servesto axially center plate stack 20 within cell 10.

While the preferred embodiments have been fully described and depictedfor the purpose of explaining the principles of the present invention,it will be appreciated by those skilled in the art that modificationsubstantiations and changes may be made thereto without departing fromthe scope of the invention set forth in the appended claims.

We claim:
 1. A pressure vessel for use in housing components of a metalgas cell comprising:a generally cylindrical container defining a firstand a second substantially circumferentially extending lip, thecontainer being constructed of a rolled metal sheet of substantiallyuniform thickness having a weld seam along its length joining togetherlongitudinally extending edges of the rolled sheet; a first hydroformedend portion; a second hydroformed end portion; and means for securingsaid first end portion to said first lip and said second end portion tosaid second lip.
 2. The vessel of claim 1 wherein said container isconstructed of a rolled sheet which is welded along substantially theentire length thereof.
 3. The vessel of claim 1 wherein the cell is anickel-hydrogen cell.
 4. A vessel for use in housing components of ametal gas cell comprising:a generally cylindrical container defining afirst and a second substantially circumferentially extending lip; afirst end portion; a second end portion; a first ring interposed betweenand welded to said first lip and said first end portion; and a secondring interposed between and welded to said second lip and said secondend portion.
 5. The vessel of claim 1 each of said first and said secondend portion is generally hemispherically configured.
 6. A vessel forhousing components of a metal gas cell comprising:a generallycylindrical container defining first and second mouth openingspositioned at each end of the cylindrical container; a first end portionsecured to the first mouth opening; a second end portion secured to thesecond mouth opening; first insert means positioned within and securedto the vessel in the vicinity of the juncture of the first end portionand the first mouth opening; and second insert means positioned withinand secured to the vessel in the vicinity of the juncture of the secondend portion and the second mouth opening.
 7. The vessel of claim 6wherein each of said insert means is a weld ring welded to the vessel,and serving to restrict movement of a plate stack adapted to bepositioned within the vessel.
 8. The vessel of claim 7 wherein first andsecond weld rings are interposed between and welded to, respectively,the first mouth opening and first end portion, and the second mouthopening and second end portion.
 9. The vessel of claim 6 wherein thecell is a nickel-hydrogen cell.